(1) Field of the Invention
The invention relates to a transmission system comprising a transmitter and a receiver, for transmitting an information signal, a video signal in particular, in a digital format obtained by means of differential pulse code modulation (DPCM). The transmitter of this system comprises a DPCM-encoding arrangement and the receiver a DPCM-decoding arrangement.
(2) Description of the Prior Art
The transmitter of a transmission system generally comprises a source supplying an information signal in the analog or digital form and which must be transmitted to the associated receiver. In a DPCM-transmission system, this information signal is first applied to a source encoding circuit which here is in the form of a DPCM-coding arrangement. This coding arrangement comprises a difference producer to which the information signal and a prediction signal are applied and produces a difference signal. This difference signal is applied to a quantizing arrangement producing a quantized difference signal. In addition, the DPCM-encoding arrangement comprises a prediction circuit having an input and an output. The quantized difference signal is applied to the input of this prediction circuit and the prediction signal occurs at its output.
The quantized difference signal occurring at the output of the quantizing arrangement is further applied to a channel encoding circuit, for example an analog-to-digital converter or a code converter, which converts this quantized difference signal into a digital channel signal which will be denoted DPCM signal hereinafter and consists of a sequence of codewords occurring at a predetermined rate f.sub.s, denoted the sampling frequency. It should be noted that the inverse quantity 1/f.sub.s will be denoted the sampling period and will be indicated by the symbol T.
Via a transmission medium, the codewords produced by the channel coding circuit are transmitted to the associated receiver, where the sequence of received codewords are converted in a channel decoding circuit into a decoded channel signal which, the transmission not being disturbed, accurately corresponds to the original quantized difference signal. This decoded channel signal is further applied to a DPCM-decoding arrangement. This decoding arrangement comprises a summing device to which the decoded channel signal and a second prediction signal are applied and which produces a sum signal. This DPCM-decoding arrangement further comprises a second prediction circuit having an input and an output. The decoded channel signal being applied to this input and the second prediction signal occurring at this output. The prediction circuit in the transmitter is of the same construction as the prediction circuit in the receiver, in order to achieve that the sum signal accurately corresponds to the original information signal.
A prediction circuit can be implemented in several different ways. Possible implementations are described in, for example, the References 1, 2, 3, 4, 5 and 6. As will be apparent from these References, such a prediction circuit is generally formed by an integrator circuit the input of which is connected to the input of the prediction circuit and the output to the output of the prediction circuit.
In the receiver, every received codeword now contributes during a predetermined time interval to the sum signal. If now a codeword is disturbed in the transmission medium, then also the sum signal is disturbed during an equally long time interval. This time interval is closely related to the time constant of the integrator circuit. If it has an infinitely long time constant (in that case the integrator circuit is sometimes called an ideal integrator circuit) then, after the occurrence of a transmission error the sum signal will never obtain its proper value again. In such a case it is customary periodically to adjust the prediction circuit of the DPCM-coding and decoding arrangement (for example at the end of every TV-line) to a fixed value.
If the time constant of the integrator circuit is chosen shorter (in that case the integrator circuit is sometimes called a leaky integrator circuit), also the said time interval becomes shorter. However, reducing the time constant results in a decrease of TV-picture quality. The highest picture quality is obtained when an ideal integrator circuit is used. If, however, a leaky integrator circuit is used, the leak factor will always be a compromise between the length of the time interval (that is to say the speed with which a transmission error is eliminated) and the loss in picture quality.
In order to yet obtain in a DPCM-transmission system using an ideal integrator circuit, that the influence of a disturbed codeword is eliminated in a very short period of time, references 7, 8 and 9 propose to add in the transmitter an error reduction signal to the DPCM-signal. This error reduction signal is then generated by an error reduction circuit to which the information signal to be transmitted, or the prediction signal is applied. In the associated receiver an error reduction signal is subtracted from the received sum signal in order to generate the original DPCM-signal. Also this error reduction signal generated in the receiver is produced by an error reduction circuit to which a signal generated in the DPCM-decoding arrangement is applied, and which, when the transmission being undisturbed, corresponds to the best possible extent to the error reduction signal generated in the transmitter.